1. Field of the Invention
The present invention relates to semiconductor circuitry. More particularly, it relates to a frequency multiplier provided with transistors for multiplying the frequencies of signals of a high-frequency such as a UHF signal, a micro wave signal, and a millimeter wave signal by fixed integer values, respectively, and also relates to a harmonic mixer provided with transistors for mixing these high-frequency signals.
2. Description of the Prior Art
FIG. 6 is a circuit diagram of semiconductor circuitry which constitutes a prior art harmonic mixer as disclosed in IEEE Journal of Solid-State Circuits Vol. 33, No. 12, December, 1998, pp. 2241, for example. In the figure, reference numeral 1 denotes a power supply terminal, numeral 2a denotes a radio-frequency signal input terminal (i.e., RF input terminal), numeral 3a denotes a local oscillation input terminal (i.e., LO input terminal), numerals 4a and 4b denote output terminals, numerals 11 and 12 denote transistors, numeral 31 denotes a constant current source, and numerals 51 and 52 denote resistors.
The operation of the prior art semiconductor circuitry will be explained. A DC voltage Vcc is applied to the power supply terminal 1 of the semiconductor circuitry. A radio-frequency signal (i.e., RF signal) input from the RF input terminal 2a is applied to the base electrode of the transistor 11, and is amplified by the transistor 11. On the other hand, a local oscillation signal (i.e., LO signal) applied to the LO input terminal 3a is input to the base electrode of the transistor 12, and is amplified by the transistor 12.
The constant current source 31 is connected to the emitter electrodes of the transistors 11 and 12. Thus, an electric current having a phase opposite to and an amplitude equal to those of an electric current flowing through the transistor 11 will flow through the transistor 12. Therefore, in the transistor 11 the RF signal of positive phase is mixed with the LO signal of negative phase and they are amplified, and in the transistor 12 the RF signal of negative phase is mixed with the LO signal of positive phase and they are amplified.
As a result, the RF signal of positive phase, the LO signal of negative phase, and mixture waves of the RF signal and the LO signal are output as a collector output of the transistor 11 by way of the output terminal 4a connected to the power supply terminal 1 via the resistor 51. Furthermore, the RF signal of negative phase, the LO signal of positive phase, and mixture waves of the RF signal and the LO signal are output as a collector output of the transistor 12 by way of the output terminal 4b connected to the power supply terminal 1 via the resistor 52.
The output signal of the semiconductor circuitry is defined as a differential signal that appears between the output terminals 4a and 4b. Thus, any (2n-1)th (n is an integer of 1 or more) harmonic having a frequency (2n-1) times as large as that of the RF signal or the LO signal has a voltage two times as large as that of the RF signal or the LO signal while any (2n)th harmonic having a frequency 2n times as large as that of the RF signal or the LO signal is suppressed and is therefore not output.
When any mixture wave of the RF signal and the LO signal is not considered, odd-order higher harmonics of these signals can appear between the output terminals 4a and 4b. The semiconductor circuitry thus operates as a frequency multiplier for generating odd-order higher harmonics.
For the mixture of the RF signal and the LO signal, any even-order higher harmonic of the mixture which is an output signal of a usual fundamental harmonic mixer (e.g., a mixture wave of f.sub.RF -f.sub.LO, where f.sub.RF is the frequency of the RF signal and f.sub.LO is the frequency of the LO signal) is suppressed, and only odd-order higher harmonic of the mixture (e.g., a mixture wave of f.sub.RF -2f.sub.LO) appear between the output terminals 4a and 4b. The prior art semiconductor circuitry can thus operate as a harmonic mixer for generating odd-order higher harmonics.
If the constant current source 31 operates ideally, neither even-order higher harmonics of the RF signal and the LO signal nor even-order higher harmonics of the mixture of them appear between the output terminal 4a and 4b. However, when the power supply voltage Vcc from which the semiconductor circuitry operates is as low as about 3 volts, the constant current source 31 cannot be manufactured as an ideal element because the power supply voltage is low. Therefore, the constant current source 31 is often replaced by a resistor of about hundreds of ohms, for example.
When the constant current source 31 does not function properly, no signal of negative phase can be generated sufficiently in each of the transistors 11 and 12. Therefore, a problem with the prior art semiconductor circuitry is that there causes an unbalance between components of either the LO signal or the RF signal at the output terminal 4a and those at the other output terminal 4b, and therefore the signal level of any odd-order higher harmonic falls, and the suppression of either any even-order higher harmonic or any mixture wave signal becomes difficult and some even-order higher harmonic signal components therefore appear between the output terminals 4a and 4b.
To enable the semiconductor circuitry to operate from a lower voltage, there is provided a method for combining the RF signal of positive phase with the LO signal of negative phase, applying the composite wave to the RF input terminal 2a, combining the RF signal of negative phase with the LO signal of positive phase, and applying the composite wave to the LO input terminal 3a, without the constant current source. In this case, the semiconductor circuitry can operate from a lower voltage because it does not include the constant current source. However, since the input signal terminals for the LO signal and the RF signal are not separated, there is a need to provide circuits each for combining the RF signal with the LO signal wave while making them be out of phase by a constant phase. It is difficult to implement such the circuits as low-loss components included in the semiconductor circuitry, and it is therefore necessary to implement the circuits as external circuits other than components included in the semiconductor circuitry.
The above-mentioned description is directed to the case where the semiconductor circuitry operates as a down-converter. If the RF input terminal is replaced by an IF input terminal, the above-mentioned semiconductor circuitry can operate as an up-converter and output an RF signal. In this case, the output RF signal can be an odd-order harmonic of the mixture of an IF signal and an LO signal (e.g., a harmonic of f.sub.IF +2f.sub.LO, where f.sub.IF is the frequency of the IF signal).
As can be understood from the above-mentioned explanation, the prior art semiconductor circuitry in which the input terminals for the RF signal and the LO signal are separately provided can suppress any even-order harmonic to be generated between the output terminals by using the constant current source connected to the emitter electrodes of the transistors that accept the RF signal and the LO signal, respectively, and can perform a mixing of odd-order higher harmonics. Therefore, a problem with the prior art semiconductor circuitry is that it is difficult to make the constant current source operate ideally when the DC voltage Vcc applied to the semiconductor circuitry is as low as 3 Volts or less. And, another problem is that when the characteristics of the constant current source are not ideal, either any even-order higher harmonic or any mixture wave signal, particularly any even-order higher harmonic of the local oscillation signal with a large power level easily appears between the output terminals.